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Does an increase in air temperature actually improve aircraft performance?
Mainly because of what pilots learn about density altitude and aircraft performance, any idea that an increase in temperature could improve performance seems counterintuitive. And indeed it is. Here's what got me thinking about it.
In studying for a written test I ran across a question phrased something like this:

An aircraft flying at a constant power setting flies from a colder temperature to a warmer temperature. What happens to true airspeed and true altitude?

The correct answer turned out to be that true airspeed and true altitude both increased. I knew immediately on the altitude because I remembered reading how very cold temperatures can cause dangerous errors in altimeters, but the airspeed didn't make sense.
I still don't understand why the increased temperature lead to an increase in true airspeed. Obviously a reciprocating engine doesn't perform better in warmer air. We don't install "interwarmers." Does it really mean that warmer air can actually improve performance? Or does this need a deeper explanation?

Maybe "improves aircraft efficiency", rather than "performance"?

"I remembered reading how very cold temperatures can cause dangerous errors in altimeters" - Be careful with that; your altimeter, by definition, shows indicated altitude, which is affected by temperature. Your altimeter will read higher than your true altitude in cold air due to the increased density (which is what's dangerous), and thus the indicated reading on the altimeter will decrease as the outside air warms. The question must assume you're also maintaining a constant indicated altitude, in which case as the air warms and your altimeter reading decreases, you'd climb to a higher TA.

The Instrument Flying Handbook outlines a scenario where the temperature is -50C and the difference between charted altitudes and corrected altitudes had to be calculated in order to conduct a safe approach. I've never been in that situation but it's good to have it in mind.

I'm open to improving my understanding of both facets of this question in future, but best I can tell the reduced air pressure makes the wing more efficient in terms of ratio of drag to true airspeed. The engine may become more efficient, but may render less total horsepower because of the reduced availability of air.

Drag (and lift) increases with density. Density decreased and so did drag. So at the same power, you can fly faster.
Now I don't know whether reduction of power of a normally aspirated spark-ignition reciprocating engine at constant throttle setting would be higher or lower than the reduction of drag. But the question says power setting.

Re "Density decreased and so did drag. So at the same power, you can fly faster."-- how do you know that Drag decreased? I think that this answer "puts the cart before the horse" -- I'd submit that you have to understand how IAS changed before you can make any prediction about how Drag changed. For more, see alternative answer https://aviation.stackexchange.com/a/95818/34686

@quietflyer I was (implicitly) comparing the same velocity. But … you are right, I forgot to take into the account the decrease in lift. Because while the same power lets you fly at higher velocity (true airspeed), it will correspond to lower indicated airspeed. And that means performance only improves if you are above the minimum drag speed.

Lets be very clear here, aircraft performance is determined by three things; weight, altitude and temperature (WAT). An increase in any one of these three things reduces aircraft performance. Drag is only considered in performance charts in respect to flap settings and other drag devices.
If you have ever calculated the takeoff and climb performance of an aircraft departing from a high elevation airport on a hot day at maximum takeoff weight you will clearly understand this.
The test question you posted is somewhat poorly written as it makes some assumptions. It doesn't mention pressure changing as you fly into the warmer air and so you have to 'assume' that the outside 'static' pressure is constant (despite the fact that, flying into warmer air would typically result in an area of lower pressure).
Re: the airspeed. Lets say you are flying along indicating 100Kts and this was also your true airspeed (this would only be the case at sea level on a standard day). As you fly into the warmer air it would become less dense. The dynamic pressure (ram air measured by the pitot tube) will reduce. Therefore the airspeed needle will drop and show a lower speed.
Let's say it drops to indicate 98 Kts... but because the power remained constant you are still doing 100 (which is your true airspeed). The question ignores the fact that engine performance decreases with higher temps., but they said 'power constant'... it's a theoretical scenario.
Re: the altimeter. Because the warm air is less dense, the static air pressure around the aircraft will decrease. The altimeter simply shows the difference between static pressure and a fixed pressure in the bellows (which can be adjusted by the setting in the Kollsman window). Because the static pressure is now lower the altimeter will indicate higher than your true altitude. You would compensate by setting the correct local altimeter setting.
Flying into the warmer air did not increase your performance! In our example above you were doing 100Kts TAS in the cooler air and you are still doing 100Kts TAS in the warmer air, it's just that the airspeed is indicating differently.
In fact, to hold the power constant in the warmer air you would need to increase fuel flow to the engine, which is actually reducing your overall aircraft performance!
Airspeed indicators show 'indicated' airspeed because this is critical for aircraft maneuvering (like rotation, approach, stall etc.).
Some altimeters have TAS dials around the edge, you set the temp. and P. Alt. in cruise they have a sub scale that will show your true airspeed. Advanced flight decks (glass) have electronic TAS displays.

True airspeed is equivalent airspeed corrected for non-standard pressure and temperature.
With a increase in temperature, TAS has no choice but to increase.  Notice that IAS did not increase.  It has nothing to do with engine performance.

@Federico How does this not answer the question?  The OP wanted to know why TAS increases and I answered it.  It is more simple than the other answers indicate.

my bad, sorry. You addressed the engine performance, but the question is about the overall aircraft one, that includes lift and drag. So, yes, you are right that this answer the question, but only partially.

A higher temperature means the molecules are moving faster. Assuming a constant atmospheric pressure, that would translate to less number of air molecules in the same amount of space.
Now, drag is caused by the airframe hitting the air molecules. There is less drag, but the power setting stays the same - i.e. the force of forward thrust is constant, but the force of drag is smaller. Airspeed increases.
Of course, with most engines, if the throttle setting stays constant, engine power is less in warm air. So the question assumes that the loss of power is compensated by an advancing the throttle.

How do you know there is less drag?  A-o-A is not constrained to be constant; in fact it cannot stay constant, unless IAS also stays exactly constant, which is not a "given" in this problem. For more,  see alternative answer http://aviation.stackexchange.com/a/95818/34686 .

Fundamentals:

The question is evidently comparing the TAS and True Altitude obtained in warmer air vs in colder air, at a given power setting and a given indicated altitude. So we are taking a "snapshot" of the airplane in steady-state flight (constant airspeed and altitude) in colder air, and another such "snapshot" in warmer air, and comparing the True Altitude and the True Airspeed.  The Indicated (not True) Altitude, as well as the power setting, is evidently presumed to be the same in both snapshots.

Weight is the same in both snapshots, and therefore Lift is also the same.

We can't automatically say "Drag will be less whenever the air is thinner".  If Lift is constant, then Drag is determined by the L/D ratio which is a function of Angle-of-Attack. If we are cruising on the "front side of the thrust-required curve", somewhat faster than the IAS for max L/D-- as would normally be the case-- then any decrease in IAS will be associated with an increase in AoA and a reduction in Drag, and any increase in IAS will be associated with a decrease in AoA and an increase in Drag.  So in the context of the parameters of this question, Drag needs to be viewed as a function of IAS, not the other way around.  We can't simply say that "Thinner air leads to less Drag which leads to more airspeed."  So this question isn't really about changes in Drag.

For the same IAS, it always requires more power to fly at a higher TAS than at a lower TAS, even through Drag is the same in either case.  Therefore if the power output is fixed and the air becomes thinner, leading to an increase in the TAS associated with any given IAS, the IAS must decrease.  (Note that this means that the A-o-A must increase.  So, assuming that we are cruising on the front side of the thrust-required curve, we do see an decrease in Drag-- just as we would if we decreased power to slow down, while increasing the A-o-A, with no change in the characteristics of the airmass.)

So we know that the IAS must be lower in the warmer air than in the colder air, and the A-o-A must be higher in the warmer air than the colder air.

At first glance, it's far from obvious that the TAS will in fact always be higher in the warmer air than in the colder air, for a given power output from the engine.  Consider this-- for a given power output of the engine, and with angle-of-attack free to vary as needed, would it be fair to say that an airplane always experiences a higher TAS in level flight at a higher altitude than at a lower altitude?  It appears that the answer is in fact "yes".  Granted, if power is constant, then at some very high altitude, the airplane will only be able to maintain altitude at the one particular IAS where the power-required is at a minimum-- but the reason for this apparent "shortage" of power is simply the fact that any given IAS is associated with a higher TAS, and therefore requires more power, at high altitude than at low altitude.  So yes, if power is constant, it appears that an airplane does in fact always experience a higher TAS in level flight at a higher altitude than at a lower altitude, and similarly, will also always experience a higher TAS in level flight at a given Indicated altitude in warmer air than in colder air.

Of course, in the real world, as an aircraft climbs above some certain altitude, the power output of the engine cannot remain constant!

Now as to your specific question--

I still don't understand why the increased temperature lead to an
increase in true airspeed. Obviously a reciprocating engine doesn't
perform better in warmer air. We don't install "interwarmers." Does it
really mean that warmer air can actually improve performance?

Only in the context of the aircraft's TAS performance for a given power output of the engine.  Not in the context of the aircraft's TAS performance at the maximum possible power output of the engine at any given altitude, or the TAS performance at any given rpm or throttle lever position.

"Obviously a reciprocating engine doesn't perform better in warmer air."
Why not? Horse Power increases with inlet temperature. Have a look at your engine handbook and look for something like "Sea Level and Altitude Performance". There should be a chart or table that makes that clear. I'm looking at the chart in my Lycoming engine manual for my Mooney, and the chart clearly shows an increase of horse power with inlet temperature.
"We don't install 'interwarmers.'"
No, we don't, but your reasoning is wrong. The premise of the question is that the air temperature changes by itself. By the way, the air pressure increase in a turbocharged/supercharged engine has an increase in air temperature as one component (although the smaller one) to provide for higher engine power. We close the cowl flaps in cruise flight to keep the engine warm, so we get higher engine output when an increase in altitude provides for cooler air to begin with!
"Does it really mean that warmer air can actually improve performance?"
yes, but not much. Air density changes (from altitude change) are much more pronounced.

When you say "inlet" are you referring to the temperature of the air entering the intake or the turbine inlet? If you mean turbine inlet that's a measure of the temperature of exhaust gas exiting the cylinder, not intake air, so I don't see how it's directly related.

you are referring to reciprocating engines in your post. There are no turbines in reciprocating engines. But then you also talk about cylinders ... there are no cylinders in a turbine engine ... sorry, I don't see how you're making sense here.

Turbocharged reciprocating engines use a turbine inline with the exhaust. When you refer to "inlet temperature" it sounds like you might be referring to TIT or turbine inlet temperature. My point was was the temperature of the air inducted and I think you were referring to the temperature of the exhaust as it exits the cylinder.

Nope. I'm not referring to turbocharged engines, and I'm not referring to exhaust. Inlet is never exhaust. Have a look at your engine handbook. My Lycoming engine manual has a chart for Sea Level and Altitude Performance and lets you compute horse power with inlet temperature as one of the inputs. Don't get hung up on the word "inlet". For most practical considerations, the inlet temp is ambient temp. That's what is the premise of your question: the plane flies into an area of higher temp. That would be ambient temp. You can pretty much assume that's the same temp that your engine sucks in.

Thanks for clarifying that you mean "air inlet" and not "turbine inlet."

I also found the chart you're referring to. Figure 3-22. http://11hc.44rf.com/manuals/engine-prop/lycoming/lycoming---operator's_manual.pdf

Yepp, that's the one. My handbook is slightly different, chart 3-21 for the IO-360-A, but I have a much older handbook.

To achieve a given HP output of a reciprocating aircraft engine an RPM setting is chosen and a corresponding published manifold pressure is set. MP has to be increased if the OAT is above standard and decreased if the OAT is below standard to achieve the published HP rating. If you have ever driven a car up in the mountains during the summer you would understand that engines loose performance in the heat.
The primary purpose of cowl flaps is to keep the engine cool during taxi, takeoff and climb and to keep an 'even' temperature over all cylinders in cruise.

Re "We close the cowl flaps in cruise flight to keep the engine warm" -- isn't the primary reason for closing the cowl flaps in cruise flight to reduce drag?  Closing the cowl flaps doesn't increase the density of the air entering the engine, so it's not analogous to flying in cooler air.


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How does pregnancy affect airline pilots operating under FAA regulations?
Female airline pilots have an area of medical concern that male pilots don't deal with: reproductive issues. If a female airline pilot operating at an air carrier in the United States of America under the Federal Aviation Administration (FAA) were to become pregnant, how would this affect her standing with a first class medical and in general with operations at most airlines?

In what country? Some countries have laws against discriminating against women because of pregnancy. I suppose this is more airline policy than federal laws. As long as the woman can pass the requirements for a first-class medical then I don't see it having any effect. The only effect is that doctors don't recommend pregnant women fly in the last part of the third trimester, and blood pressure is probably the biggest medical issue.

@RonBeyer Good call. I narrowed the question down to a discussion of the matter in the USA under the FAA. Do you think it's still too broad?

@RyanBurnette That narrows it down to one regulatory body, so it should be fine.

There was an article in the NY about this yesterday:  http://www.nytimes.com/2016/08/17/business/when-the-captain-is-mom-accommodating-new-motherhood-at-30000-feet.html?hp&action=click&pgtype=Homepage&clickSource=story-heading&module=first-column-region&region=top-news&WT.nav=top-news&_r=0

@RonBeyer Many countries that do have such anti-discrimination laws also have exceptions for when safety could be affected.

As long as the pilot is having a first class medical, her pregnancy should have no effect on the operations, at least legally, though the regulations differ from country to country.

I'm not aware of any FAA regulations that prohibit pilot ing during pregnancy. The closest it comes is in 14 CFR § 61.53 - Prohibition on operations during medical deficiency, which says (noting that pregnancy is not a medical deficiency in any way and the regulation is taken here in the widest sense) that:

(a) Operations that require a medical certificate. Except as provided for in paragraph (b) of this section, no person who holds a medical certificate issued under part 67 of this chapter may act as pilot in command, or in any other capacity as a required pilot flight crewmember, while that person:
(1) Knows or has reason to know of any medical condition that would make the person unable to meet the requirements for the medical certificate necessary for the pilot operation

As long as the pilot feels that she is medically fit and has a valid medical certificate, they can't be legally prevented from flying the aircraft.

In case of other regulattions (like EASA), pregnant women are not allowed to pilot after 26 weeks of pregnance. For example, UK Aviation Authority says:

(c) Pregnancy
(1) In the case of pregnancy, if the AeMC or AME considers that the licence holder is fit to exercise her privileges, he/she shall limit the validity period of the medical certificate to the end of the 26th week of gestation.  After this point, the certificate shall be suspended.  The suspension shall be lifted after full recovery following the end of the pregnancy.
(2) Holders of class 1 medical certificates shall only exercise the privileges of their licences until the 26th week of gestation with an OML.

There are a number of medical issues that the pilot should be aware of while pregnant, so that appropriate measures could be taken, like,

Ability to move the controls to their full extent.

Ability to wear seat belts.

Effect of high altitude operation on the foetus
among others. The airline policy will vary and they have to consider a number of other conditions- for example, the ability of flight crew to egress during a crash etc.

I know at least one GA pilot who considered her pregnancy a "medical deficiency" such that she could no longer act as PIC until after she gave birth (she got "big enough" that she couldn't get full up-elevator without the yoke hitting her abdomen) - that's obviously a case-by-case situation though, and depending on the airplane might not be an issue (e.g. an Airbus or a Cirrus with a side-stick)

Here is a decent article from AOPA on the matter (covers the topic from an FAA/USA standpoint). Looks like legally they don't have to say anything and they can't legally be pulled

The law is on their side, too. In the early 1990s, the U.S. Supreme Court ruled that unless a woman voluntarily disclosed her pregnancy to her employer, she couldn't be switched out of her job to one that carries less risk to her fetus. Essentially this means that a woman has the right to decide when to declare she is pregnant to her employer, and to stay in her position even if it may cause harm to the developing fetus.

It also seems like the regulations vary from airline to airline,

Few airlines today ban first trimester flying (from the time of conception through the first 12 weeks of pregnancy). Most, along with the military, permit flying through the second trimester (24 to 28 weeks) during an uncomplicated pregnancy. A few companies have no written policy and handle their pilots' pregnancies and leave issues on a case-by-case basis.

The article goes on to cover  most of the topics related to this pretty comprehensively and is worth a read.
Of course there are some general medical concerns that any mother should take into account.

I am a pregnant pilot with an ATPL in Australia where we are allowed to legally fly until the end of our 30th week. I'm currently at my 27th week and definitely still feel fit to fly, I just had my Class 1 medical Renewal and my Doctor said he would be more than happy to sign me off to fly for longer but the CASA regulation is firm. I myself still feel fit to fly and am lucky that I have crew that load bags and do the heavy lifting so to speak.

Welcome to aviation.SE and congratulations! Unfortunately yours is a personal anecdote that does not really apply (the question is about FAA regulations, you mention Australia's CASA), while we are looking for factual answers, like extracts and links to relevant regulations.

This is not specific to aviation, and I don't know the rules for countries other than Germany.
I was just starting to argue about radiation doses, but this turns out not to be an issue in most cases:
An average pilot / FA receives a dose of about 2 mSv / year (worker in nuclear power plant: 1mSv/y). An embryo / fetus is not allowed to collect more than 1mSv during the entire pregancy. This would not happen if the much larger mother collects 2mSv in one year, but there are still special precaution requirements. For example, the received dose has to be measured weekly instead of monthly.
But our law gives even more. For example, after the 3rd month, a pregnant woman is not allowed to work on any means of transport, which an aircraft definitely is. And in general, pregnant woman are not allowed to do a job where they have to lift / push / move weights over 5kg permanently or 10kg occasionally.
Due to this laws, pregnant women have to be grounded immediately, no matter what the airline says.

Since a typical pregnancy is 9 months, during which the last 6 weeks they would definitely not fly, this means at most the pregnant woman would get 1.25mS according to your numbers, so I'm not sure I would consider radiation exposure as a larger factor. I'm not sure about Germany, but I know in the US most women don't get radiation tested weekly during pregnancy...

@RonBeyer: No, the dose is indeed not that high I thought it were, but it isn't. But that doesn't change the laws about monitoring. I'd also like to know if flying staff is tested in general  in other countries. But as I wrote, it turns out they are not allowed here to push a trolley through the aisle...

@Ikosatu: I think, what I wrote was a little misleading. The unborn child should not collect more than 1mSv within the 9 months. Flying staff collects an average of 2mSv, the maximum is typically at about 5mSv per year. This dose is a total body dose, not per kilogram. The simple math is: If a 70kg woman collects 5mSv per year, 3.5kg of her body collect 0.25mSv per year or 0.1875mSv in 9 months. In reality, it's more complex since there are some weights on different tissue types, but the fetus also doesn't weight 3.5kg all the time...

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What is a Second Segment Weight Limit?
I am very interested in the principles of take-off performance optimization.
During my research I found an interesting picture, showing the software used by a European carrier.
However, the output presented shows a value I do not understand:

In the lower right corner, the following values are shown:

PTOW: 68.5
Field Length: 73.2
Sec. Segment: 71.0
Obstacle: 68.5

I do not understand what the Second Segment limit stands for.
I know about the different phases of the take-off run, however I am not aware how the Second Segment could be limited by any other weight than the Obstacle Clearing weight.

What does PTOW stand for?

@Jimmy Performance TakeOff Weight

Related: https://aviation.stackexchange.com/questions/3310/how-are-take-off-segments-defined

The second segment of the takeoff is defined as (source: skybrary.aero):

Second Segment - begins when the landing gear is fully retracted. Engines are at takeoff thrust and the flaps/slats are in the takeoff configuration. This segment ends at the higher of 400' or specified acceleration altitude. In most cases, the second segment is the performance limiting segment of the climb.

The minimum climb gradient is defined for each segment of the takeoff (same source):

Each segment of the one engine inoperative takeoff flight path has a mandated climb gradient requirement. For example, a gross second segment climb gradient capability of 2.4%, 2.7% or 3.0% is required for two, three and four engine aircraft respectively. Similarly, the required gross gradients for the fourth segment are 1.2%, 1.5% and 1.7% respectively.

I'm not familiar with the performance tool you show, but I would assume that the given value is the maximum TOW, which still ensures this required climb gradient can be met with one engine inoperative, so in your example a maximum of 71.0t allows 2.4% gross climb gradient for the second segment.

OBSTACLE LIMITED CLIMB: It is natural to think of a required gradient to clear an 'Obstacle'. But that gradient arises from the geometry of the runway, obstacle height/distance and flightpath. The closer, and/or taller the obstacle, the higher the gradient required to clear it with the required margin. Interestingly, it should be pointed out that the required gradient will depend on the actual liftoff point ass the distance to the obstacle would change. 2nd Segment is a different matter . . .
SECOND SEGMENT LIMITED CLIMB: is about the capability of the airplane to maintain a minimum climbing flight path almost immediately after liftoff, with engine out, and gear up, takeoff flaps and takeoff thrust AND flying V2. This is governed by density altitude. It is equivalent to the WAT (Weight, Altitude, Temperature) charts on smaller airplanes where rwy length is rarely a factor. That gradient of 2.4% for twins has to be met no matter where in the world the rwy is located.
2nd segment can become the limiting factor at high temperatures and higher altitudes. This can become limiting even for a 10 to 12,000 ft RWY at merely 1000 ft elevation on hotter days.

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On FSX flight lesson on climbs and descents, what does "free ride" mean?
The following is from a lesson on Microsoft Flight Simulator X regarding climbs and descents:

While engine power moves a car uphill, gravity pulls it down. Without your foot on the accelerator, the car's downward speed is determined by the steepness of the hill it's descending. The steeper the hill, the faster it goes. If the hill becomes shallower, then the speed decreases. If the hill becomes too shallow, then some power is necessary to maintain sufficient forward speed.
Airplanes can also move downhill without power.
Just lower the nose, and you'll get what appears to be a free ride (it isn't, but let's not get into that).

I cannot seem to understand what the author meant by the last sentence. Is the author trying to say that descents are not "free" as in descents are not "powered" by the weight of the aircraft and some form of thrust is needed?
I am rather new to the concepts of the physics of flying and may be missing out on something. Hopefully, someone is able to point out what I am not catching. Thanks so much!

I think how it flies can greatly help your understanding.

If I remember correctly most of those FSX lessons were done by Rod Machado.   He’s a good CFI but has a tendency to speak tongue-in-cheek about things. What he means by that is that by lowering the nose you can maintain the same airspeed but use less power to do it i.e. getting a free ride out of it. In truth what’s happening is that the airplane is compensating for the reduction of thrust by exchanging potential energy for additional kinetic energy, and it does this by descending.  It’s not exactly a “free ride” per se - energy is conserved after all - but the effect is similar to a car rolling down a hill whereby the driver does not need to give the engine gas to maintain a given speed.

Aircraft can trade altitude for speed and vice versa. Probably that's what the author meant (but only he knows for sure).
I find the quoted text misleading for people without prior knowledge of aircraft dynamics. Lowering the nose does not necessarily descend the plane (although most of the time it does).
Please get a copy of "Stick and Rudder" by Wolfgang Langewiesche. It's an excellent book, eye opener, and fun to read.
One of the main points of this book is that - opposite to beginner's intuition - the "elevator" (stick) controls the speed of the plane along its path, and the engine thrust controls the vertical speed (ascend/descend) of the plane. Actually it's a bit more complicated than this. That's why I recommend the book.
A try to clarify your comment, although I'm afraid it will only confuse you more:
Descending means loosing altitude. Altitude means energy. That energy has to go somewhere. Nose down means trading that energy gained from altitude into kinetic energy (speed). Since the plane must not exceed the maximum (or required) speed, this kinetic energy must be dissipated. That happens through drag. Drag increases with speed, and speed will increase when pointing the nose down, thus dissipating energy faster. Thrust only increases the kinetic energy (speed) of the plane, instead of dissipating it.

Thanks so much for your reply! Is it logical for me to interpret it as such: Descents require more than just the weight of the aircraft i.e. some form of thrust is needed for the descent (either from engine power or trading altitude for thrust). Hence, it is not "free"?

No, disagree. You don't need thrust to descend.

In a nose up descent, thrust is needed isn't it?

No. What makes you think so?

http://fap.if.usp.br/~vvuolo/A%20-%20HOME%20-%20Fisica/FMT0458/air%20resistent%20forces_files/four-slow-descent.png
I cannot seem to understand how the aircraft in the above link can still continue its flight path (indicated) after removing the thrust portion from the above image

Refering to your image: the vector-sum of all forces is 0. That remains true when the thrust is zero. The lift is perpendicular to the direction of flight. Direction of flight is forward/downward, therefore the lift is upward/forward. That forward component of the lift is what keeps the plane flying forward along this direction of flight.
If still not convinced, think about gliders (without engine).

BTW, your last comment together with the image would be a nice question on its own!

Thanks so much for your replies! I think I have a better understanding now! Will check out the book!

To elaborate, (perhaps this is obvious, but...) if engine thrust in the descent went from greater than zero to zero, something would change.  Either airspeed would decay, or if airspeed was maintained with the nose, the glide path angle would steepen.


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How to reduce the noise of the supersonic passage(bang)?
as you know the transonic phase is often associate with the bang.
The principal cons of the transonic phase is the noise,it's why there is not supersonic passenger aircraft( cause the end of the concorde).Nasa (x59) and Boom(xb-1) do research to reduce this noise at door slam.
How is it possible? I know that we need a aerodynamic plane respecting the aera rule and have a thin fuselage. Have you" wind tunnel" images?or theorical explications?

Edit: for those who want i found more information about the bang on this vidéo. (2:35 to 4:45)

Beautiful picture!

Transonic flight produces only localised shockwaves, the boom only occurs once true supersonic flight is achieved.

Related: Is there a boomless sonic technology in any state at this time?

The noise is hardly the principal reason for the lack of supersonic passenger aircraft.  The primary reason is cost: there aren't all that many people willing (or able!) to pay ~30 times as much as a subsonic plane to save a few hours.  For instance, Google suggests that the Concorde's trans-Atlantic flights averaged only half full...

Most supersonic aircraft have points where the cross-section suddenly changes, such as the fuselage nose, the wing root leading edge or the wing trailing edge. The points of sudden change produce sharp changes in air pressure, i.e. loud sonic booms. Concorde was one example.
By designing the plane's cross-section to vary smoothly from end to end, the handful of points of major change are replaced by many more points of minor change. The booms are thus reduced in loudness and spread out in time, making them less intrusive.
Other factors which help include overall aerodynamic efficiency, light weight and high cruise altitude. A small, carefully-profiled and aerodynamically modern high-flying aircraft can thus be a lot quieter.

These changes help in mitigating the local shocks for drag purposes, but all supersonic vehicles experience oblique shock that produce bang when hitting the ground; how is that reduced/removed?

@JZYL At supersonic speeds the local shocks tend to extend and coalesce into a smaller number of travelling oblique shocks. By mitigating the local shocks one can also mitigate the travelling shocks. The bang cannot be wholly prevented as long as aerodynamic lift is required, but it can be made less intrusive.

If you are not afraid of UFOs, you can check MHD principles and plasmas.
The idea is to interact with the ambient fluid to improve the flow, thus reducing the shockwave
Real results are very limited and it requires too much power to be airworthy.

Can you explain mhd please

If the fluid can be turned into a plasma - ionized gas, then the mhd (magneto-hydro-dyanmic) drive can generate a magnetic field to interact with it.
It is reported as "impractical" as a boat engine in 1965 on wikipedia. However there are more recent studies done by China's University regarding modern jets physics but I don't know the outcome.

So you say that to reduce noise we can(maybe) turn fluid in ionized gaz ?how this have a impact on the sound? What is the caracteristics of ionized gaz better than normal air?

Speaking as a sometime electromagnetics engineer working on plasma generation and control, I do not see how MHD can help greatly. Technically one might be able to use it to smooth out the airflow - if you can stop the plasma corroding the airframe, which is in fact your biggest theoretical challenge. Reprofiling the airframe is a lot easier.


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How to identify shock waves and expansion waves on a representation of the mach number?

This image is a representation of the Mach number for $M_{\infty}=2.0$
I am very new to CFD and high speed aerodynamics. I'm trying to analyse this image, specifically to understand why the velocity decreases behind the trailing edge.
On the following image, I've tried to identify shock waves (1 and 3) and expansion waves (2) when the direction of the flow changes. However, after an expansion wave, the velocity should increase and it is not the case here so my interpretation must be false. How?

Thank you

Your airfoil is traveling supersonic at around Mach 2.

First the flow encounters a bow shock. Away from the leading edge, the bow shock morphs into an oblique shock. You can clearly see Mach number rapidly decreasing after the shock.
After passing through the thickest point of the airfoil, the expansion wave begins due to gradually increasing expansion angle. You can see Mach increasing, all the way up to around Mach 2.2.
At the trailing edge, the flow encounters a compression angle if you consider the wake as a boundary. Here, an oblique shock is encountered. Since oblique shock is less powerful than a normal shock, you can clearly see that Mach isn't decreasing as much as after the bow.

why is there a shock wave at the trailing edge at all? from the view of air molecules, they are 0 mach. Suddenly the object bumps into them with mach2, air molecules can't get out of the way, so there is shockwave. There are air pinned in the bow area, so the shockwave doesn't happen at the surface of the object. If there is a trailing edge shock wave, it must mean: the parasitic boundary layer air on the object is traveling faster than mach1 with respect to air in region (2) in the above diagram.

@eliu "from the view of air molecules, they are 0 mach"? You're thinking from an external observer pov, which isn't really relevant here. With respect to the airfoil, the free-stream is traveling at Mach 2. Likewise, immediately adjacent to the airfoil boundary, the flow is standing-still with respect to the airfoil, even though sitting on the ground you'll see it moving at Mach 2.

I am trying to understand this from observer's pov. It at least enhances my understanding. As it emphasize the fact that is the air that was dragged alone by a plane crashing into stationary air. That's fundamental cause of the shockwave. And the stationary air was accelerated in such sudden way caused the immense drag and sound barrier. But I think your answer confirmed my "crashing" idea.

Expansion begins right after the bow shock due to the concave contour of the airfoil. At the thickest point ambient speed is regained already and the flow accelerates further. All of the surface is the source of one big expansion fan. Also, it would be nice to mention the boundary layer as it is clearly visible.

Would anyone wish to consider doing the analysis with a supersonic airfoil design.  This data seems to show a lot of drag at Mach 2.  Where would the CP be with that particular airfoil?


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What are the fatigue regulations in the US and Australia for military transport helicopters/rotorcraft?
For transport category military helicopters, what are the fatigue regulations as specified by the US and Australian regulatory bodies?
I've managed to find that FAA has 14 CFR 29.571 (Fatigue Tolerance Evaluation of Metallic Structure) but this could just be for civil helicopters.

No. It is a question because I'm finding it difficult to find the above stated information.

'Relevant regulations' is very broad.. there's manufacturing, ongoing airworthiness, operations, licensing, noise.. the list goes on and they are often integrated or combined with fixed-wing regulations. What exactly are you looking for?

Ben thanks for the clarifications. I'm particularly interested in the regulations related from a structural airworthiness viewpoint.

@Raptor, perhaps you should edit your question to only ask about Australian CASA regulations. That part of your question is specific and answerable. Your question under Q1) a) seems too broad, since a proper answer would list every applicable regulation from every possible regulatory body worldwide.

Ok Everyone, I've made the question super specific and look forward to your thoughts. Thank you all for your feedback and helping me to sharpen my question. Best.

Hello Everyone, I've edited the question considering all your feedback and comments. Could you all please help me to reopen this question? Best.

Hi Federico, Thank you heaps for voting to reopen my question. @mins also helped me with the editing and sharpening of my question which resulted in the final form as presently posted (Thanks @mins). I look forward to the help from others to reopen. Best.

Hi Everyone, I will appreciate your feedback and comments on my question. Also, if I need to repost this question, then could you please let me know how to do this. Best. Raptor

Fatigue is mandatory. It’s the military. That’s why they call your uniform fatigues. It’s present tense.

DASR is the regulatory authority for Australian military aircraft. The question is unanswerable because they don't make this information available to the public

The US Federal Aviation Administration does not regulate aircraft owned and operated by the military services. I suspect that is the same situation with Australia's CASA.
Parameters for aircraft service life for military aircraft are defined by the manufacturer. The aircraft is designed to meet the requirements specified by the agency requiring the aircraft. This is usually expressed in flight hours, cycles or a combination of that and possibly other parameters.

"Parameters for aircraft service life for military aircraft are defined by the manufacturer". Incorrect. The regulatory authority is DASA, not CASA in this situation. That info is not made public

@Craig No. My statement is correct. You only took one sentence and used it out of context. The second sentence addresses your point.

The manufacturer makes suggestions but the operator and regulatory authority (DASR) define the limits and can actually extend life limits if you mod in strain gauges for instance, or carry out ongoing inspections like NDT. E.g. on Hawk 127 BAe says x hrs, but RAAF decided to implement extra systems (HUMS, NDT, etc.) and now extended life beyond original design life.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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How can a stowaway hide in the undercarriage?
I'm curious about how and where a stowaway could hide in the undercarriage of a plane.
According to this news report:

The man, believed aged 24, was found in the undercarriage of the
  plane and was taken to hospital.

Wheel wells are deceptively large spaces when the gear is down. It is fully possible to climb up and hide inside one. It's a very different story when the gear is retracted. On many aircraft, the gear doors are  closed when the gear is not in motion, making the space look bigger.
They are fairly easy to access through the landing gear with a little bit of climbing, or if low enough (like the example below) just jump up into.

737 Wheel Well, Petr Volek, from airliners.net
A graphic about a similar event in Hawaii a few months ago:

 From the Independent
Gear stowaways are not new and there have been plenty of occurrences over the years, unfortunately most result in fatalities.

Gear bays are unpressurized, hence normally very cold with air too thin  to sustain life.
Gear bays are very cramped when the gear is up. If you're in the wrong spot, you will be injured.

It is when the landing gear is lowered (and doors opened) for landing that the fatalities occur, assuming they have not already expired from the environment during the flight. Unable to hold onto the airframe and stay off moving parts they (very gruesomely) fall out minutes away from landing. This is also the story that media often picks up.

So, presumably the hypothermia protects against hypoxia making this potentially survivable?

@CountIblis that's a question for http://health.stackexchange.com/

@CountIblis This high altitude climbing article suggests the opposite: "The combination of low barometric pressure, cold temperatures and high winds that are common in high-altitude environments can compound hypoxic physiological stresses, resulting in an increased risk of hypothermia and frostbite" Source

@CountIblis see the case of Ewa Wiśnierska


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What is the history of the "Superbay" at San Fransisco International?
The Superbay is a maintenance hangar on the north end of the field at San Francisco International Airport, north of runway 28R and west of runway 19R. On the airport diagram it is not labeled, but next to fire station no. 2.
It is currently used by United Airlines and American Airlines for maintenance. Ground control refers to it as the "Superbay" when giving instructions.
When was the Superbay built? Did it have any uses before the current tenants? Where did the name come from?

Superbay Hangar is a maintenance facility at San Francisco Airport.
About previous tenants and when it was built, this forum mentions that:

It is the large aircraft hangar built by AA in the 1970's to house the
  747. Both AA and TWA were the first tenants. Now it's UAL and AA. UAL uses it as a line maintenance hangar for the wide-body fleet.
It sits all alone out towards the bay. When VIP's come to SFO, they
  park next to this hangar due to its isolated location.
  ...
  The SuperBay was built and is owned by the City of San Francisco. It
  is primarily used by UA/AA.

Going through the timeline on SFO's History, the vacant area of this hangar is visible in one of the pictures of 1960s. In later pictures (1981), it appears to be there (shown in the background)1.
Interestingly, more references are found about this hangar in the future development plans of the airport than the history. Examples can be seen here (PDF, p20 and 28) and here (PDF, p6). This site also talks about some future work for Superbay Hangar.

1: I am not 100% sure about direction of the camera, but it might be pointing towards that hangar.

Thanks for the info, I hadn't seen the SFO history timeline before. I'm hoping there are more details out there.


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Should an aircraft's registration match with its tail number?
What are the requirements for an aircraft's tail number to match the registration?
What happens if an aircraft changes its number? Is it grounded until the new number can be painted on?

The registration number painted on the aircraft is required to match the aircraft's registration certificate (and airworthiness certificate). If the numbers don't match the aircraft's paperwork isn't in order, and it can't legally be flown.
There is a procedure for changing your N number which is usually used to put a "special" (reserved) number on an aircraft:

Send the FAA a letter requesting an N number change.
(It probably helps to have a good reason - like say United Airlines buying a used jet and wanting to re-register it with a N___UA tail number from their reserved block).
Wait for the FAA to send you paperwork authorizing the N number change.
You will get 3 copies of form 8050-64 - two for the FAA, one for you.
Put the new N number on the plane.
Usually you'd do this by repainting the aircraft (or the portion of the tail where the number is), but you could also do vinyl stickers or something similar)

One of the copies of form 8050-64 becomes your temporary registration certificate at this time, and you must carry that along with your original registration certificate showing the old registraton number.
Within 5 days of applying the new N number return one of the copies of form 8050-64 to the FAA to obtain a new registration certificate with the new N number.
Within 10 days of applying the new N number return the other copy of form 8050-64 to the FAA to obtain a new airworthiness certificate with the new N number.

  I expected to find something stating how long you have between the time the FAA grants the request to change registration numbers and when you must apply the new one to the plane, but as far as I can tell you can take your time doing this: there doesn't seem to be an expiration on the authorization to change N numbers.

As long as you comply with the requirements for the registration number change the aircraft is only "grounded" for as long as it takes to apply the new registration number (if you're repainting this could take a while ; if you apply a large sticker with the new number on it it could be done in less than a day).

Does the FAA ever grant exemptions from this? In the UK, many ex-military aircraft are granted exemptions to display their military serials instead of their civil registrations. (See, for example, Avro Vulcan XH558.)

@gsnedders UK law is rather different. 558 is still on the military registry, not civilian. So its registration is still XH558. Dutch procedures are more in line with what you seek. There for historical aircraft the old code can be shown accurately, the new "modern" code printed in smaller print somewhere on the aircraft (still visible but unobtrusive, for example it might be on in small print under the old code).

@jwenting XH558 is registered with the CAA as G-VLCN and operates under a permit to fly issued by them. She has an exemption to display her old RAF number.

And notably she doesn't, as far as I'm aware, have G-VLCN anywhere on her. Hence her markings do not in any way match her registration.

@gsnedders Waivers are available for special circumstances (like filming), but as far as I'm aware an aircraft on the civilian registry in the US must display appropriate registration markings. There is a standing exemption allowing "small N numbers" on warbirds though, and it sometimes takes a sharp eye to spot them.

I think you're also asking about Airline ship numbers (or fleet number, etc). All airline aircraft in the US (and probably most other countries) have to have a registration, and most airlines also have a ship or fleet number. This is the number that's approved on the airlines OpsSpec (Operations Specification, that basically outlines the entire airlines operations, from Diversion airports to numerous other considerations). If what I understand is correct, an airline just lists the aircraft N number, with it's internal number (whatever they want it to be, sometimes a part of the registration, other times completely different). As long as it's documented, and any changes are documented correctly, it doesn't matter how they correlate.
In the image below, you see the ship numbers on the nose door. The registration would be near the tail, and there are FARs for sizing and placement of that. Also, it's rare that an airline will change the registration or fleet number.

I think the question is actually asking about whether the registration number painted on the tail has to be correct, and how soon the registration number has to be repainted if the aircraft is re-registered with a different number.

Sorry for not being more clear, this wasn't intended to be part of this question. However, I have wondered about this before, thanks for the info. If you want I can ask a new question so this answer has a proper place.


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How often do people fly SVFR?
I was going over Special Visual Flight Rules (SVFR) with my instructor recently and he told me that he has only requested it a few times, and that it is mainly used if you are flying from deteriorating weather into improving weather. In other words, you are sure that where you are going is clear. I got thinking about it and felt that if there were ever an SVFR situation, I would most likely ground the flight (until I get my instrument rating that is).
That being said, how often do pilots request SVFR?
It seems like it would cause people to end up in situations that they can't handle i.e. weather deteriorates more instead of getting better, does this happen often?
It has been mentioned that SVFR rules vary from country to country, I am asking about peoples experiences in the USA but am happy to hear of other similar situations around the world as well.

You will most likely not get a good reply, as SVFR differs from country to country and there may be several reasons to request SVFR. Germany recently changed VMC in control zones and will see a rise in SVFR requests, but I am not aware of any sources to quantify how many there are... The second question would be easier to answer...

My apologies I did not realize the ambiguity, I guess my question is related to SVFR in the US ill edit it to reflect this.

Thanks for the edit, I updated the question with the [tag:faa-regulations] tag, so further answers will take this into account. I'll leave my answer below however as well.

Here is a case where a pilot requested an SVFR clearance in deteriorating weather and the resulting crash ended up on National News:  https://en.wikipedia.org/wiki/Jessica_Dubroff

From my experience in the US at the facilities I've worked, and the discussions I've had with other controllers, it depends heavily on the area's weather patterns. The facilities I've worked, it's been usually a tool, when people are trying to get in right before the weather hits, or they're seeing good improvement in neighboring airports and at the main one, and know that it'll be good enough to make it between the two areas.
Other places, like coastal or areas where fog banks are common, SVFR can be useful, esp if only half the field is covered in clouds, and you need to make it in or can easily get out.
Personally, I'd probably just file IFR and cancel when I got to a good spot VFR, if I just wanted to go out and enjoy, versus just flying to a destination.
Also, note, SVFR in terms of separation is only one SVFR aircraft(and really aircraft period) in the airspace at a time. So, you'll be delayed to come in if there's IFR, or other SVFR aircraft ahead of you.
Helicopters generally do request it more than fixed wing aircraft in my experience. Also, realize a pilot has to explicitly request SVFR, a controller can't prompt or hint at the correct terminology to use to request like saying "Do you have a SPECIAL request"

Is it now the case that an SVFR departure is not allowed if an IFR craft is inbound? The reason that I ask is that back in the 1990s we used to expedite our departure by using SVFR to avoid having to wait for an arriving/departing IFR aircraft at non-radar airports. We would typically fly out from under the stratus in a few miles, climb until we knew center would be able to see us, then pick up our IFR clearance. This was especially useful for commuter operations at Wenatchee, WA and Redmond, OR.

Correct, No SVFR in the airspace if there is an IFR aircraft at or inside the airspace, so usually you'd terminate SVFR operations when an IFR aircraft is ~15 miles from the airspace to allow enough time to get the aircraft in or out of the airspace before the IFR arrival gets in.

I've done it once, or maybe twice in 1200 hours of flying.
The first time was when I was stuck at an airport in the LA basin which was below standard VFR minimums due to how thick the smog was. I filed SVFR to take off, and by 5000 feet I was above the smog layer and it was blue skies the rest of the way home.

Copters fly SVFR more often than fixed-wing for a number of reasons: SVFR minimums are lower (see 91.157), can be done at night, and are not subject to the "NO SVFR" of Part 91 Appendix D Section 3 that prohibits SVFR at large Class B airports.
Also, Copter instrument approaches can terminate at the MAP with an SVFR clearance to the airport. Such approaches may even be noted on the flight plan. For example, the MAP for JFK COPTER RNAV (GPS) 028°(PDF) depicts various VFR routes to the NYC heliports. These can be flown SVFR.

Deleted my answer and gave you the upvote, thanks @rpb!

The below was posted before the OP clarified in his question that he was interested in FAA-based data and anwers.

I am not aware of any statistical data, but I can answer the second question in your text.
Remember that SVFR is not only about visibility, but also about staying clear of clouds. Airspace D in Germany requires cloud distances of 1.000ft vertically and 1.5km horizontally. You are also required to remain at least 500ft above ground during flight, except for landings and takeoff.
If your cloud base drops below 1.500ft, you could technically no longer maintain VMC, even though maybe your visibility was more than 10km. In this case your only option is to request SVFR.

I've flown SVFR a lot. Like requesting access to military routes (MR on your area maps) it seems to be something that lots of people don't use but those who know how to use them discover they are great!
I have found that traffic control (especially the non-tower sites you address as e.g. "Radio Hawthorne") love hearing these requests and are eager to help you figure out the details you need to get what you want - probably because it's a bit more interesting than most of the requests they hear :)
Caution, Opinion ahead:
I highly recommend you learn to use it and memorize a few of the versions more relevant to your location (e.g. "SVFR to VFR on top" to get above spotty low clouds). You'll sound like a pro and traffic control will usually be quite friendly and receptive about it. In fact I can't recall ever being turned down on such a request, although I have received advice to change my strategy once or twice (including once when I was guided into a channel to see about a dozen military craft go by at close enough range to count mustaches). SVFR is a fun and powerful tool for the general aviation pilot and an excellent way to build skill. Do them with your instructor too while you have the chance because they are indeed a skill that is worth working on.

Why would you want to fly on a military route?  While legal, its greatly increasing the risk of a mid-air.

@Rhino Driver - Quite to the contrary, I find they are usually completely abandoned and avoided because using them involves contacting a controller and asking permission and people have been conditioned to think they must be busy. As a practical matter I think they are very nearly always empty, at least I've only ever encountered traffic on one ONCE and that was when I was flying straight over an air base on what turned out to be air show day. Otherwise they are just nice empty patches of sky in my (anecdotal) experience.

Its been my experience that most near mid-airs I've heard about involving military and civilian aircraft have occurred in MOA's or MTRs when we are task saturated and unable to see.  At 350+ kts from 200-500 AGL, the majority of the pilots time is going to be spent clearing terrain and making designated target times.  Often, terrain masks the presence of other aircraft leading to dangerously close passes--such as when I rolled over the top of a mountain and suddenly had a HUD full of aircraft in front of me.  In this case I passed close enough to see the faces of the crew.

@Rhino Driver - interesting, sounds like you might have been in the military aircraft? I usually open by asking if the MR is clear and have assumed if they tell me "yes" that it means no scheduled flights. Whereabouts are your experiences from. On the West Coast there are thousands of miles of MR with what seems like very little activity on them. Do you think that most MR flights are pre-planned and in the system so flight operations would know about them or no?


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What is the difference between a propeller and a turbo propeller?
I have found the differences rather interesting between the Beechcraft Bonanza engine and a turboprop, I have heard there the same and they're different. But I need a straight answer.

By "Propeller turbine" do you mean Turboprop?  Just asking for clarification, as I've never heard the term...

Do you mean turboprop (as in turbine engine) or turbocharged piston? This question certainly parses but it makes almost no sense to me nonetheless.

You may want to edit the question to say something like "turboprop v. a piston prop", after the selected answer.  It will make the question easier to find for later users.

I think you are trying to ask

What is the difference between a propeller and a turbo propeller?

A traditional propeller plane is powered by an reciprocating (piston) engine. These can be in the form of radial, flat, or V style engines with a few other configurations used here and there.

On the other hand a turbo prop is a propeller plane that is powered by a turbine engine. In this instance the shaft of the turbine is used to drive a propellor. One could think of this as putting a propellor on the front of a jet engine.

The bit about "internal combustion" engine is off. Both types are internal combustion engines, because in both cases the combustion chamber is part of the working fluid flow circuit. All kind of jet (tubrofan, turbojet, ramjet, scramjet) and rocket engines are also internal combustion engines. External combustion engine would be e.g. steam engine, but those don't have the efficiency needed for aircraft propulsion.

I would agree that in both cases the combustion is internal. In this case I am using the colloquial version of "internal combustion engine" to refer to piston engines.

@JanHudec: Sometimes they do!


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How to correct for crosswind in VOR holding pattern without wind information?
I've heard many articles about crosswind correction in VOR holding pattern. They mainly tell the outbound leg=correction times three things for slow speed aircraft. I notice all of them require wind information in holding pattern. What if somehow under some circumstances the pilot doesn't have the wind information whatsoever in holding pattern? Like the wind may change unexpectedly. Is there any method to maintain precise 4 minutes holding pattern in such condition?

One you are established inbound on the VOR radial note the heading you are flying to maintain that course. The difference between that and the radial is your wind correction angle. Multiply it by 3 and apply it to the outbound course heading.
To do this you don't need to know anything about the wind.  You just need to be able to intercept and track the inbound radial.
You can repeat this every inbound leg so if the winds are changing you can update your correction every 4 minutes and you always know you are on course one of ever four minutes. With the generous obstacle protection area even small errors will not compromise safety.

So if I am inbound on R360, with a right crosswind, and my heading is, say, 010 inbound, you want me to fly  180-(10*3)=150 degrees outbound? That is way too big a crab.

@rbp yep. Your turns and outbound leg account for 3/4ths of your hold but you are only correcting for the wind on the outbound leg and not the turns. You won't track parallel on the outbound but the idea is to make up for the drift in the turns so when you end the turn inbound you are roughly on course. It's a rule of thumb of course and use your results along with the next inbound leg to recalibrate for the next circuit.

I guess with a large xwind,using a std rate turn I would undershoot the outbound leg on the outbound turn, and overshoot on the inbound turn. So having extra room would compensate. Thanks. I must have forgotten.

The protected area on the holding side is actually a little "fatter" near the inbound turn at the end of the outbound leg to account for this exact situation.

When holding, it is true that you should take then inbound wind correction and multiply it by three and use that for the outbound correction.
Your question centers on what happens if you don't know the correct wind information and obstacle clearance.  Fortunately for us, the FAA has given us a wide obstacle clearance area for the holds.  While it is important to stay on the protected side of the hold, if you find yourself on the unprotected side of the hold, you are still "protected".
Picture taken from http://forums.jetcareers.com/threads/holding-pattern-protected-area.80377/

The idea is to fly the correct entry procedure and it will help to ensure you stay within the obstacle clearance protected area.

A direct entry requires passing the holding point and immediately turning to the outbound leg.  You may not know the wind correction so fly the outbound heading.  Upon turning inbound get back onto your inbound course and try to determine your wind correction angle

A parallel entry requires passing the holding point and tracking outbound on the radial.  You should be able to figure out your wind correction.  Turn 180° + 45° to re-intercept the inbound course.  At this point, you should have a good idea where the wind is coming from.
A teardrop entry requires passing the holding point and turning 30° into the protected side.  When you turn inbound, there should be enough time to figure out what the winds are doing.

If the winds are constantly changing, do your best to stay on the protected side of the hold, if you venture a little to the unprotected side, you are still covered.

I am not sure the FAA obstacle clearance areas are relevant to someone presumably flying most of the time in the eastern hemisphere. Be very careful here since US - TERPS has a number of deviations from the ICAO PANS-OPS resulting in very different obstacle clearances. Apart from that note, the procedure you describe is clear.

I can't help feeling that this doesn't really answer the question. You're essentially saying "get your entry right, figure out the wind, and don't panic if you're not precise", all of which is good advice but it doesn't directly answer the question about how to determine the wind correction angle.

For lateral corrections, use a trial an error process of crabbing to maintain the selected track to the fix.  One the airplane maintains a stable track on the inbound leg, note the crab angle and make use of that, applying the same angle inbound to the fix and an the same angle in the opposite direction on the outbound leg.
To compensate for unknown headwinds and tailwinds, fly a consistent indicated airspeed and time your outbound leg from and inbound leg to the fix.  Note the difference between intended and actual times to fly the leg eg you’re flying a 1 minute leg but it takes you 1 minute and 40 seconds from rollout on inbound leg to crossing the fix, there’s a 40 second difference.  If your leg takes longer than planned at constant airspeed you have a headwind, if it’s shorter, you have a tailwind.  Take half that difference and subtract that time for your outbound leg for inbound headwinds and add for inbound tailwinds.  Eg as in the example above, difference was 40 seconds longer, so it’s a headwind.  Half of that is 20 seconds so you should fly an 1 min - 20 seconds = 40 seconds long outbound leg so your next inbound leg is one minute long.


text:
How does the start and ignition of aircraft gas tubine engines work?
2a) How does the APU produce the required starting air supply.
b) How is the starting air supplied and controlled from the APU to the engine
c) Describe how the torque from the starter motor is transmitted to the engine rotating assembly.
d) Describe the main functions and components of a high energy ignition system.

Is this an exam question?

2a] how does the APU produce the required starting air supply.

The APU generates pneumatic power using bleed air extracted between the compressor and combustion chamber.

b] how is the starting air supplied and controlled from the APU to the engine

The starting air is supplied to the engines starters through a network of ducts and valves.

c] describe how the torque from the starter motor is transmitted to the engine rotating assembly.

Usually the starter motor turns the accessory gearbox, which rotates the jet engine shaft.

d] describe the main functions and components of a high energy ignition system.

An high-energy ignition systems obviously rely on igniters which supply electrical sparks in the combustion chamber for starting or maintaining the air/fuel combustion.
The igniters are powered by exciters generating high-voltage pulses from the aircraft electrical power system.
The exciters are controlled by the engine controllers according to the position of the fuel control ; ignition and fire switches.

Bro Thank You Soo much @user9138


text:
Radar Control; is it possible for ATC to instruct track instead of heading?
Is it possible to use the track instead of headings in the aim to apply a radar separation?

related

Radar controllers take wind into consideration when issuing headings - "Fly heading one six zero".
Issuing a track would not be practical. This puts the burden of figuring out what course to fly on the pilot. Not all aircraft have a glass cockpit or instrumentation necessary to easily, quickly, and accurately compute a heading to fly to stay on course. The radar controller knows real-time information, knows the 'big picture', and has practice issuing headings.
Actual position is used to provide separation. Heading doesn't matter for separation.

"Issuing a track would not be practical." No, it wouldn't be impractical, it would be impossible. By definition, track is what the plane flew over ground. You cannot assign what to have done in the past.

@Andreas, in this case ‘track’ means direction of track and that can be assigned. But it can only be flown by GPS and controllers can't assume aircraft have that.

Well, I certainly understood what was meant, but that doesn't mean it was written what was meant. We have to go by what was written and cannot interpret around at will. And "direction of track" would mean you extrapolate the track (from the past), and ATC doesn't assign that. "... and that can be assigned". No, it can not. If you disagree, show a source, or I won't continue this discussion. What was meant ... I think the op just misused the term, he/she meant course. ("Is it possible to use the course instead of headings ...") and just didn't know that what he/she thinks of is course

No, ATC will not issue instructions to aircraft to maintain a specific track. From ICAO Document 4444 (PANS-ATM):

8.6.5.1    Vectoring shall be achieved by issuing to the pilot specific headings which will enable the aircraft to maintain the desired track.

If a controller wishes an aircraft to fly a specific track toward/away from a VOR navigational beacon, they may instruct pilots to fly a certain VOR radial, which is essentially a track. However, this only works if flying to or from a VOR or similar beacon.

I guess then that SIDs and STARs should be banned altogether since they do not comply with ICAO.

SIDs and STARs are not vectoring. The question is specifically about ATC issued instructions (radar vectoring), not about ATS routes (SIDs, STARs, airways, RNAV routes etc.)


text:
What are the costs of 1 hour flight in modern low-cost airlines?
Thinking about "low cost" airlines that often tickets of about 40 Euros for 2 flight hours, I was wondering how these companies can cover their costs.
Considering for example EasyJet that fly with Airbus A319 (150 seats), with a unique cost of about 90 million dollars, on a typical 2 hours flight (common average European route) every flight could gain about 6000 Euros in tickets.
I can't understand where is the "gain" considering fuel, crew costs, maintenance etc...
Can you give some references?

The "gain" is usually in the extra's. Pay for luggage, pay for carry on's, pay for snacks, pay for drinks, pay for premium seats, pay for priority boarding, etc. They also don't usually buy brand new aircraft, they pay their workers poorly, and some have "interesting" maintenance practices.

Well... I generally see only some drink or snack... how much gain? 500 Euros???? It seems already too much....
And... EasyJet buy its aircrafts, there are dozens of its orders placed in Airbus

Baggage is a lot, for example on a short flight I looked up on EasyJet 20kg of luggage runs you 32EUR, 32kg of luggage is 104EUR. There is an admin fee of 14EUR for booking. Extra 3 EUR to 13 EUR to pick your seat (per leg). Extra 1% fee for paying by credit card. So a short flight that was advertised at 50 EUR both ways ended up being over 100 EUR just after adding one 29kg bag and picking seats.

Ok, you are right about Extras but for example me, my friends, my family... we never add any extras on these flights. Are we the unique ones? or maybe the exeptions are who have additional fees?

Don't forget that not every passenger onboard has paid the same price. Thanks to yield management, the price you pay will vary greatly depending on various factors. Some passengers might pay a lot more, thus increasing airline revenue.

Even if their profit margin was 1000 euros per flight. They might fly that route 5 times a day, near-on 7 days a week multiplied by 100 routes.... thats not bad. But I betcha their profit is a lot more than 1000/flight. The economics must add up, otherwise we wouldnt have low-cost carriers.

The cost of flight is a quite good question. You may also find that many income comes from other sources (resales of the aircraft, subventions, food/drink, maybe cargo transportation,...)

In addition to all of the fees that others have mentioned, keep in mind that the average ticket probably isn't selling for 40 EUR. Probably closer to ten times that figure when last-minute tickets are factored in. Also, the airlines are not paying sticker price for the planes. Larger airlines (like easyJet) are typically getting very large discounts by ordering many planes.

This article presents the full breakdown of the operating costs of an A320.
TL;DR: about 15kUSD/hour, including market-based depreciation. The scenario is based on a private-jet scenario, with much fewer hours per year than your typical EasyJet aircraft, so this number should be considered as the upper limit of the actual operating costs of a low-cost airline.

Please note (or keep in mind) that private planes are subject to far more relaxed inspection and overhaul regulations than commercial aircraft are and that drastically changes the cost to operate.

There are a lot of factors that account for this. This answer covers the bulk purchase aspect of the deal. Basically EasyJet and the like see cost benefits by only flying a singly type of plane and thus only need mechanics and pilots trained on that air-frame and spare parts are interchangeable etc.
They also see discounts, as mentioned in the comments by not offering things like meals, free luggage etc.
Since they make short haul hops between nations often they may be able to work out deals for hauling cargo as well as people to undercut the cost of your ticket.
I cant find the article but it has been noted that in many cases by buying 100 planes at a time significantly reduces the cost of each air frame. They then in turn sell them in a shorter period of time and don't fly them to the end of their life. In many cases an air frame thats only a few years old will hold good value considering the sever discount it was original purchased at.
You can find some info on the cost of operation for a long haul flight here. Things like fuel consumption rates etc and the like will carry over to short haul flights.

Ok, thanks all for your answers but I think that is missing the most important information: how much for one flight hour on this kind of jets? What are costs? Please, do you have this kind of info? I think that is necessary for further considerations about prices, gains, etc

This covers the answer for long haul flights which will share some of the same costs. Most of that info is not public and it can be hard to determine objectively


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Are approach and departure procedures based on visual reference points official VFR procedures?
Pilots flying VFR follow clearances like the "mall visual" or "east channel departure." These are based on features of the local area that are easy to identify.
Are procedures like this official? Where could a reference be found for these procedures?

Which airport are you referring to?

Alaska Supplement has many VFR departures like that, which are outlined in the section for an appropriate airport. Some TACs have preferred VFR routes (corridors) on them.

Yes, they are official, this is the KSEA MALL VISUAL RWY 34R/C/L approach plate:

I can't find a "East Channel Departure" but if you know what airport it belongs to, you can find it in the Terminal Procedures publication, either in paper form or online.
They are not VFR procedures, but rather visual IFR procedures. The MALL VISUAL would not be assigned to a VFR flight. Some airports do have special arrival/departure procedures which apply to VFR flights; these can be found in the Airport/Facilities Directory (A/FD) or on a website like http://www.airnav.com/ (for the US anyway).

In some cases these VFR arrival/departure routes are listed and mapped in the back pages of the A/FD.

@fooot: The procedure is visual because it requires visual contact with ground and/or appropriate landmarks to be safely flown, but is IFR because it can be used for issuing IFR clearances. Most commercial flights always operate IFR from ground to ground even if weather would be visual.

People use IFR/IMC and VFR/VMC very interchangeably, when they're not. This is an example of Instrument Flight Rules in Visual Meteorological Conditions.
But, named departures aren't only IFR. I know of at least one airport, San Carlos Airport in California, that has named departures for VFR flights.

@Jungroth The procedures you mention at KSQL are unofficial (as far as the FAA is concerned) noise abatement procedures, sponsored by the airport. They are not official FAA / National Airspace System terminal procedures, and not to be found in the TERPS or A/FD. A visiting Part 91 VFR aircraft would not be expected to know these procedures or be able to follow them.

I believe the "East Channel Departure" is from KRNT... which is right near KSEA.   Oddly, I flew in that area for several years, and didn't know those are "Official"; I was under the impression that such local terms were just for convenience of local pilots.

@fooot: IFR is not the same as IMC:  IFR is a set of rules you fly under.  IMC means you can't see out the window.  It is entirely possible to be flying under IFR rules, but with perfect visibility.  Hence, "Visual IFR" makes sense.

@abelenky I think you're right about Renton and the East Channel (unofficial, for local convenience). But the Mall Visual at Sea-Tac is official. Basically check the TERPS for official IFR procedures.


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Why might a Cessna 208 need a FAR Part 23 waiver?
The capacity of the Cessna 208 is stated as 9 passengers, or up to 14 with a FAR Part 23 waiver. FAR Part 23 states that it applies to any aircraft with "more than 10 occupants and that is intended for use in operations under Part 135" (commuter or on-demand operations).
What part or parts of Part 23 require the C208 to have a waiver?
Is it common for waivers like this to be issued for the C208 or in similar situations on other aircraft?

it's in the question: just the pax+pilot capacity will require a waiver when it is intended to be used for commuter or on-demand ops

FAR 23 requires commuter-category aircraft to have at least two engines. Other categories have a limit of 9 passengers IIRC.

@Qantas94Heavy why don't you add this as an actual answer, and fooot should approve it.

The Cessna 208 is a bit of a confusing model when it comes to seating.
Short version:
Under US legislation, the aircraft may have up to 11 seats and can carry 9 passengers and 2 pilots. Other countries did recertify the aircraft to have up to 14 seats. By doing so they adopt the FAA certification based on part 23 and add text to waive the 9 passenger limitation.

Long version:
Originally the type was approved by the FAA in October 1984 under FAR 23 (up to amendment 28) and given FAA Type certificate number A37CE. The aircraft is certified as 11 Place Closed Land Monoplane (PCLM) and later as 11 Place Closed Sea Monoplane (PCSM).
In 1984 the applicability of Part 23 was described by 23.1 (since amendment 10) as:

(a) This Part prescribes airworthiness standards for the issue of type
  certificates, and changes to those certificates, for small airplanes
  in the normal, utility, and acrobatic categories that have a passenger
  seating configuration, excluding pilot seats, of nine seats or less.

So this is where the 9 passenger limitation originally comes from. This also implies that if you operate with only one pilot, one of the 11 seats must be unoccupied.
Until 1971 there was no mention of a 9 passengers limitation and since 1987 (amendment 34) a new category of commuter planes is defined allowing for more than 9 passenger (up to 19 passengers) in propeller-driven, multiengine airplanes.
Since the Cessna 208 is a single engine aircraft, the commuter category cannot apply. As a result, when the aircraft is operating under the FAA type certificate, there is a limit of 9 passengers.
When the aircraft is registered and operated outside the USA, the FAA rules do not necessarily apply. Usually a country adopts the Type Certificate of the original certifying country, but changes can be made. This is exactly what happened with the Cessna 208.
Since the aircraft offers space for more seats and the maximum take-off weight allows for extra passengers, Cessna started to offer a 14 seat model for use outside the USA.
For aircraft with more than 11 seats, the flight manual has a supplement stating:

Limitations
Utilization of the 13-place, 14-place and Utility seating options
  described in this supplement is limited only to nations where approval
  has been received. The United States does not authorize more than 11
  seats in the airplane. In the 11- seat configuration, the right front
  seat may be occupied by either a second crew member or a passenger,
  However, when the right front seat is occupied by a passenger, only
  eight seats in the aft cabin can be occupied.

Europe (EU)
The EASA Type certificate allows the 11 seat model 208. It also mentions a 14 seat model 208B being approved by the UK CAA. When EASA took over aircraft certification in Europe, the UK CAA approvement for 14 seats was adopted, but only for the 208B model.
In section 2.E (only for 208B):

A 14 seat place configuration was certified before 28 September 2003
  by CAA UK (see UK TCDS FA54) and subsequently adopt by EASA. Those
  aircraft have to be operated i.a.w. FAA approved AFM 1329-3- 13PHUK
  and AFM Supplement D1340-1-13UK or latest approved revision.

The original UK CAA approval included the text:

MAXIMUM OCCUPANTS/PASSENGERS: 14 occupants, including pilot. Refer to
  British Aeroplane Flight Manual

Australia
The type certificate for the Cessna 208 mentions:

Australian registered Cessna 208 aircraft may be configured for
  greater than 9 passenger seats. The requirement of FAR 23 paragraph
  23.1, introduced by amendment 23-10, to limit the passenger seating configuration, excluding pilot seats, to nine seats or less is not
  applicable to Australian aeroplanes certificated under Section 101.22
  of Civil Aviation Orders.

In the type certificate for the Cessna 208B reference is given to the UK CAA approval for more than 11 seats.
New Zealand
New Zealand allows the 208 to be operated with 14 seats.

In New Zealand the Model 208 Caravan has been approved for operation
  in the fourteen seat configuration in accordance with the provisions
  of the Fourteen Place Seating Supplement to the Model 208 Pilot’s
  Operating handbook. (A similar Supplement is included in the Flight
  Manual for the 208B.) The 208 aircraft was “accepted on the basis of
  compliance with FAR23 excluding the wording of FAR 23.1(a); ‘that have
  a passenger seating configuration, excluding pilot seats, of nine
  seats or less’.”

South Africa
The type certification document from New Zealand mentions that at the time of first approval (1985) Cessna stated:

Australia, UK, South Africa, Norway, Sweden, Kenya and Columbia had
  all accepted the Cessna 208 with more than nine passenger seats.

Interestingly in February 2014 the CAA of South Africa (SACAA) denied that the 208 was ever approved for operation with more than 11 seats in South Africa. Operators using the Cessna 208 with more than 11 seats found themselves operating the aircraft illegally suddenly.
Quickly a Supplemental Type Certificate (STC) was approved in June 2014 which allows operation with 14 seats again.

Cessna 208 is certified for FAR 23 from amendment 23-1 to 23-28 for up to 9 passengers and crew.
Section 23.3 (d) allows for up to 19 passengers and crews was amended in 2012 in amendment 23-62 however it requires multiengine plane and not applicable to the 208.
Hence 208 is not qualified to fly more than 9 passengers without the waiver.
Additional reading material:
Cessna brochure suggest that passenger limit is 14 with appropiate waiver and not compatible with aircraft operate on US registry.

In addition to the requirement for multiple engines, the Caravan may also not meet the specs for emergency exits (FAR 23.807) and aisle width (FAR 23.815) that kick in from 10-19 passengers.


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How do aviation GPS navigation systems report GPS accuracy?
While looking over a tutorial on how to operate the Garmin GNS430, I couldn't find any info on how the device would report a degraded or low accuracy signal. What are the accuracy requirements for aviation GPS navigation systems and how are they required to alert a user in the case of a reduced accuracy situation?
I'm not specifically asking about RAIM here, as I believe that is mostly designed to identify a malfunctioning satellite or compromised signal. An example scenario would be some atmospheric condition that reduces positional accuracy to 1/4 mile or less.

The required GPS accuracy depends mainly on the application. En-route requirements are more lenient than the accuracy requirements for non precision approaches.
The GPS accuracy is the result of inaccuracies in the signal combined with a factor based on the geometry of the satellites. Within the receiver the position error is thus estimated by looking at

The geometry of the satellites contributing to the position fix
The estimated user equivalent ranging error (UERE) caused by various disturbances

From the geometry of the satellites, the (Geometric) Dilution of Precision (DOP) is calculated. This is a number that expresses how the geometry of the satellites used to calculate the position influences the accuracy of the position solution. In an ideal situation, $DOP=1$, any value less than $4$ is considered good.

The DOP can be split up in three components, the horizontal (HDOP), the vertical (VDOP) and the time (TDOP). For horizontal position accuracy HDOP is used.
The second factor in the accuracy estimation is the User Equivalent Ranging Error (UERE). In the illustration above, this is the width of ranging bands.
UERE is composed of various factors:

Ephemeris data - Errors in the transmitted location of the satellite
Satellite clock - Errors in the transmitted clock.
Ionosphere - Errors in pseudorange measurements caused by ionospheric effects
Troposphere - Errors in pseudorange measurements caused by tropospheric effects
Multipath - Errors caused by reflected signals entering the receiver antenna
Receiver- Errors in the receiver's measurement of range caused by various internal factors.

The effect of the ephemeris data error is in the order of 2 meters (standard deviation).
The accuracy of the satellite clock used to be intentionally degraded under the Selective Availability programme. SA was turned of in May 2000, drastically improving the GPS accuracy. Receivers developed before that date are unaware of this change and will estimate a worse accuracy than they actually achieve.
. The residual clock error after SA was turned of is equivalent to about 2.5 meters (standard deviation).
The GPS signal includes parameters for an ionospheric model which can be used to correct ionospheric effects. The residual error cause by signal delay in the ionosphere is in the order of 5 meter (standard deviation).  It is the most significant contribution to the UERE.
Tropospheric effects are much less than the ionospheric effect. The effect is about 0.5 m (standard deviation).
Multipath effects (caused by reflections of signals) are very much depending on the environment. In a city the effects are worse than in the air. For airborne applications multipath can be cause by reflections of the signal from the wings. The effect is very limited. On the airport surface there can be significant effects especially near buildings.
Receiver errors caused by unstable oscillators (thermal noise), quantization and rounding errors, software errors etc. This contribution can be reduced by improving the design of the antenna and receiver; which all comes at cost.
Let's estimate the contributions to UERE about 3 meters.
The UERE is the root mean square of all these contributions.
$$UERE=\sqrt{2^2+2.5^2+5^2+0.5^2+1^2+3^2}\approx6.75meters$$
The 95% accuracy is then estimated by $2\times{UERE}\times{HDOP}$. For a HDOP of $2$, the position would be estimated to be within 27 meters 95% of the time. Typical accuracy is better, because the UERE factors are estimated quite conservatively.
Augmentation systems such as WAAS and GBAS can improve the GPS accuracy by sending correction signals. The GNS 430 (without the 'W') does not use these signals.
If the DOP is greater than $4$, the GNS 430 will display a message "Degraded accuracy". You can find the DOP on the Satellite status page. This page also shows the estimated position error.
Note that all the calculations to estimate the position error assume that GPS is working as it should, i.e. a fault free condition. Faults can occur in the satellites, in the transmission of data to the receiver and in the receiver itself. Fault detection is part of the RAIM function.

Wow....  I'm almost scared to ask, but does any of this change with WAAS?

Using WAAS the UERE will be reduced, and so the accuracy is improved. Also the integrity (fault detection) benefits.

Needed to add, the higher you are, the better vertical accuracy you get; meteospheric events influence the refraction significantly, and the refraction mostly influences the vertical measument, not the horizontal one.


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Is it permitted to deviate from yellow lines, during taxi?
During taxi procedures, the pilot should follow the yellow lines, to avoid a collision with any obstacle or even with another aircraft. Is the any situation, which the pilot can deviate from the yellow line at their own discretion?

Not just permitted, but sometimes explicitly encouraged for large aircraft.

Some taildraggers do S-turns down the taxiway to be able to see in front of them.

There's no rule that you have to follow the lines, they're there purely as a guide, a reference for pilots to know where the center of the taxi lane is. Following it is at a pilot's discretion, to be deviated from at any point. Generally you follow the line because it's easy and it should keep you out of trouble, common reasons for deviating are:

Obstructions: vehicles, people, debris
Clearance: airplanes with large wingspans may need to hug one side of a taxiway to get around parked airplanes or buildings. In a small airfield you'll often have to get around a parked airplane or two


text:
Do concurrent operations on a single runway occur anywhere else besides AirVenture?
At the EAA's AirVenture fly-in in Oshkosh (KOSH), landing and sometimes takeoff operations are very often done by dots which are painted on the runway, making it so that multiple operations (a landing or a takeoff) are done at the same time on a single runway.
A diagram of those dots for those who don't know what I am talking about:

Image Source
Does this kind of runway operation (i.e. concurrent landings or takeoffs on a single runway) happen anywhere else or is this special to AirVenture?

Are you including military operations in this, or only civil ones?

@Pondlife Just civil

ICAO defined procedures for Reduced runway separation which to some extend permits concurrent operations on a single runway. ICAO does not mention anything about coloured dots, but the ICAO procedures do open up for doing something similar to what you are describing.
The following is from Doc 4444/PANS-ATM:

7.11.4    For the purpose of reduced runway separation, aircraft shall be classified as follows:

Category 1 aircraft: single-engine propeller aircraft with a maximum certificated take-off mass of 2 000 kg or less;
Category 2 aircraft: single-engine propeller aircraft with a maximum certificated take-off mass of more than 2 000 kg but less than 7 000 kg; and twin-engine propeller aircraft with a maximum certificated take-off mass of less than 7 000 kg;
Category 3 aircraft: all other aircraft.

[...]

7.11.7    Reduced runway separation minima which may be applied at an aerodrome shall be determined for each separate runway. The separation to be applied shall in no case be less than the following minima:
landing aircraft:

a succeeding landing Category 1 aircraft may cross the runway threshold when the preceding aircraft is a Category 1 or 2 aircraft which either:

has landed and has passed a point at least 600 m from the threshold of the runway, is in motion and will vacate the runway without backtracking; or
is airborne and has passed a point at least 600 m from the threshold of the runway;
a succeeding landing Category 2 aircraft may cross the runway threshold when the preceding aircraft is a Category 1 or 2 aircraft which either:
has landed and has passed a point at least 1 500 m from the threshold of the runway, is in motion and will vacate the runway without backtracking; or
is airborne and has passed a point at least 1 500 m from the threshold of the runway;
a succeeding landing aircraft may cross the runway threshold when a preceding Category 3 aircraft:
has landed and has passed a point at least 2 400 m from the threshold of the runway, is in motion and will vacate the runway without backtracking; or
is airborne and has passed a point at least 2 400 m from the threshold of the runway;

departing aircraft:

a Category 1 aircraft may be cleared for take-off when the preceding departing aircraft is a Category 1 or 2 aircraft which is airborne and has passed a point at least 600 m from the position of the succeeding aircraft;

a Category 2 aircraft may be cleared for take-off when the preceding departing aircraft is a Category 1 or 2 aircraft which is airborne and has passed a point at least 1 500 m from the position of the succeeding aircraft; and
an aircraft may be cleared for take-off when a preceding departing Category 3 aircraft is airborne and has passed a point at least 2 400 m from the position of the succeeding aircraft.

(note: I have changed formatting slightly to better fit this format)
These general rules are interpreted different by different countries, but some different forms of reduced runway separation are used in different places of the world.

While this is a good summary of the regulatory limits simultaneous operations on a single runway, it's not at the level of the KOSH special flight procedures detailed in the annual NOTAM. At KOSH, it's not unusual to see multiple aircraft over the runway or a plane touching down on the purple dot with a plane passing overhead targeting a touchdown on the pink dot. Intersection departures are handled by flagmen who wave them off between gaps in arrivals. Watching arrivals is it's own show.

AirVenture arrivals do not follow any rules other than what is printed in the NOTAM. For a few days each year, KOSH is the busiest (by operations) airport in the world, easily surpassing even KATL, and part of it is using the dots for simultaneous landings on different parts of the same runway. Watch some videos on YouTube and you'll be both horrified and amazed.

Does this kind of runway operation (i.e. concurrent landings or
  takeoffs on a single runway) happen anywhere else

That is a very broad question, and the answer is, not surprisingly, "yes".  For example, I know of a glider club that operates from a very wide, 7000' long runway that was originally built for military flight training during the second world war.  It is not uncommon to have ten gliders lined up in position on one side of the runway, and a towplane taking off with a glider in tow, and a returning glider landing on the other side of the runway and well behind the lined-up gliders, all at the same time.
Perhaps the question was meant to refer specifically to takeoffs and landings under some sort of air traffic control guidance, but it didn't actually specify that.


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Are there any aircraft designed to operate between an altitude of 50-80km?
Aircrafts operating at high altitude falls into several categories:

Wings with high aspect ratio (Global Hawk, TR-1, Scaled Composite Proteus,...)
High velocity aircraft (Concorde, A-12, X-51,...)

Aircraft falling into those two categories cruise between 15km and 30km high.

Balloons (up to 50km high)
Suborbital aircraft (X-15, Space Ship Two), performing their mission between 80 and 120km high.
higher resides low earth orbit, and thus satellites which can hardly be qualified as aircraft.

I cannot find any aircraft designed to perform its mission at an altitude between 50km and 80km, which corresponds more or less to the mesosphere. Is there any aircraft designed to perform its mission into this 30km wide slice of atmosphere? What is the usefulness of this slice for aviation?

Is there any aircraft designed to perform its mission into this 30km wide slice of atmosphere?

(between 50km and 80km) - apart from some experimental rocket planes and so on, I can't think of any. The main problem is there's so little air - very little opportunity to generate lift, and very little air to run a jet engine.
In theory it should be possible for a winged aircraft to fly up there (it is below the Kármán line) but the engineering difficulties would seem to make this impractical.  For more detail I'd recommend the top answer for this question: What determines the maximum altitude a plane can reach?

What is the usefulness of this slice for aviation?

If it were possible, it would be useful for futuristic or military things like:

spy planes like the SR-71 but higher and faster
super-fast suborbital transport
passing through on getting into orbit - single stage to orbit
vehicles for example.

(All of these examples are pipe dreams at the moment of course.)

Thank you for your question, I never "realised" that even most "exotic" airplane have a maximum ceiling height of not more than 30 km (stratosphere). Why should that be?
Starting from very first principle, a force shall balance gravity, in "conventional" aircraft, lift equal weight:
$$ W = \frac{1}{2} \rho S V^2 Cl $$
Let's have a look at what happens to density ($\rho$) for height above 30 km, it goes almost to zero! So above this "threshold" it is more convenient to use other principles to contrast gravity, buoyancy, like baloons, or a force directed vertically, similarly to missiles and thurst!

How would balloon buoyancy work if there is not enough atmosphere for "conventional" lift?